The present invention relates generally to the field of sand cast molding. More specifically, the invention relates to a process and apparatus for recovering molding media in a foundry, and the process for using the recovered molding media in the foundry.
Green sand casting is a well-known process for forming cast metal articles. In this process, a casting mold for making castings, formed from molding media that is primarily sand and bentonite clay, is used in only one molding cycle for the production of one or multiple castings. Once the casting solidifies in the mold, the mold is broken down and the casting cycle is complete. A portion of the molding media can be recycled for another casting process, however, much of the molding media exits the foundry as foundry waste. In the U.S. alone, foundry waste accumulates at a rate of approximately 6 to 10 million cubic yards per year. The large volume of foundry waste coupled with the increasing cost of landfill acreage and transportation is problematic.
In Green Sand Foundries a casting mold is made using a xe2x80x9cgreen sand moldxe2x80x9d that defines the external body of the casting and a xe2x80x9ccorexe2x80x9d that is placed inside the green sand mold to define the internal configuration of the casting. FIG. 1 is a process flow diagram illustrating the well-known manner in which molding media is used to form green sand molds and cores used in a casting cycle within a green sand foundry. Prime (i.e. new) silica sand of input stream 1 and the chemical binder of input stream 3 are used to produce cores in core-forming step A. The core, which must withstand high pressure during formation of the casting, is made by coating the particles of sand with any one of a number of chemical binders, such as for example a two-part urethane system, and which are well known in the art. The sand/chemical binder mixture is pre-formed according to the internal configuration of the casting to be made and the chemical binder is then reacted to complete a high-tensile core. Prime silica sand 2, bentonite clay 4 and organic additives 5 are used to produce green sand molds at mold-forming step B. The green sand mold is made by press forming sand that is coated by a mixture of bentonite and organic additives, generally known as xe2x80x9cbond.xe2x80x9d The addition of water of input stream 6 hydrates the bond and causes the grains of sand to adhere to one another and take shape. The green sand molds typically comprise by weight, from about 86% to 90% sand, 8% to 10% bentonite clay, 2% to 4% organic additives and 2% to 4% moisture.
After the core and green sand mold are formed the core is inserted into the green sand mold and molten metal is poured into the green sand mold to produce a casting at casting step C. After the molten metal solidifies, the casting undergoes xe2x80x9cshakeoutxe2x80x9d at shakeout step D to break apart the green sand mold and the core into small particles or clumps. During shake out the particles of the core flow out of the solidified casting and become commingled with the particles from the green sand mold. A portion of the materials that once made up the green sand molds and core, represented by output stream 7, are recycled to make green sand molds at mold-forming step B for a subsequent casting cycle, and an excess portion of the materials that once made up the green sand molds and core, represented by output stream 8, exits the process as xe2x80x9cmolding waste.xe2x80x9d The addition of prime sand 2 at mold-forming step B compensates for the xe2x80x9cfinexe2x80x9d sand that is taken out of the process after each casting cycle. Prime bentonite clay 4 and prime organic additives 5 compensate for the additional bond needed to coat the uncoated prime sand and also the uncoated sand that once made up the cores. The addition of prime bentonite clay and organic additives also compensates for molding media loss due to high temperature exposure.
The excess molding media, that is, foundry waste which cannot be reused for subsequent casting cycles, is generated at several locations within the foundry. The composition and particle size distribution of foundry waste can vary depending upon the areas of the foundry in which it is collected, but foundry waste can be generally classified in two broad categories, namely, xe2x80x9cmolding wastexe2x80x9d and xe2x80x9cbag house dustxe2x80x9d. The term xe2x80x9cmolding wastexe2x80x9d refers to the excess molding media from broken down green sand molds and cores, output stream 8, produced during shakeout. Another source of foundry waste, represented by stream 9, is generated by defective cores that never get used in the casting operation. Molding waste can include materials present in both output streams 8 and 9, as well as molding media which fall from the conveyor system at various stages throughout the foundry. In many green sand foundries, the molding waste typically contains by weight from about 80% to about 90% sand, from about 6% to about 10% bentonite clay and from about 1% to about 4% organic additives. Molding waste includes sand that is coated with bond as well as individual particles of sand, bentonite and organic additives.
Attempts have been made to reduce the accumulation of molding waste by mechanically removing the bond from the sand so that the sand is sufficiently clean to be reused in the production of cores. In such processes the sand is recovered, but the bentonite clay, which costs several times more than sand on a weight basis, and the organic additives are discarded. Another disadvantage of mechanical reclamation is that the cost of prime sand is sufficiently low in many geographic areas that the capital investment for sand recovery is economically unfeasible.
Another large source of foundry waste, stream 10, includes fine particles of sand, bentonite clay, organic additives and debris collected in the foundry""s air evacuation system. Foundry waste 10 is commonly known in foundries as xe2x80x9cbag house dustxe2x80x9d. Bag house dust contains substantially more bentonite clay than does molding waste. Bag house dust typically comprises from about 40% to about 70% sand, from about 20% to about 50% bentonite clay and from about 10% to about 30% organic additives.
In some cases, certain foundries have been able to recover bentonite clay by introducing the bag house dust back into the water system that is used for making green sand molds in the casting process. In this manner, the bag house dust is mixed into the water system treated according to the advanced oxidation process (AO technology) and is placed into a settling tank. See, Advanced Oxidants Offer Opportunities to Improve Mold Properties, Emissions; Modern Casting, September, 2000, p. 40-43. Upon settling, water containing bentonite clay is pulled from the top of the settling tank and reused in the green sand molding lines. A disadvantage, however, is that the sludge which settles out of the settling tank and is discarded contains most of the sand in the bag house dust.
Accordingly, there is a need to reduce the amount of foundry waste exiting a green sand foundry. There is also a need for a process to recover sand that has sufficient quality to be used in the foundry to make cores and green sand molds and which can yield quality castings in a subsequent casting process. There is also a need for a process to recover sand, bentonite clay and organic additives to decrease the amount of prime materials that enter the foundry as raw material.
These and other needs are addressed by the present invention which is based on the recognition that much of the sand and bentonite clay contained in foundry waste derived from a typical green sand foundry can be recovered for reuse in making new green molds by a two-step hydraulic separation procedure which first recovers coarse sand suitable for reuse in making new green sand molds from the waste and thereafter separates out fine sand unsuitable for use in making new green molds from the remainder of the waste to produce an aqueous byproduct bentonite clay stream that can also be used in making new green molds.
Thus in one embodiment of the invention, bag house dust, after slurrying in water, is hydraulically separated to produce an underflow output stream containing at least about 40% of the sand originally contained in the bag house dust as well as an aqueous overflow stream containing at least about 60% of the bentonite clay in the bag house dust. In accordance with the present invention, it has been found that the relatively coarse sand contained in the underflow has a particle size distribution allowing it to be directly used for making new green sand molds for a subsequent casting cycle. Accordingly, this coarse sand product is recycled to the green mold preparation station, after optional removal of water, for reuse in making additional green sand molds. The aqueous overflow stream produced as a byproduct of the first hydraulic separation step, if desired, can be subjected to a second hydraulic separation step to remove most of its sand content. This sand is too fine to be useful in making additional green sand molds and is therefore discarded. However, the effluent output stream produced as a result of this second separation step, which contains at least about 50% of the bentonite clay originally found in the bag house dust but very little sand, can also be directly used for making new green sand molds and accordingly is also recycled to the green sand molding station for this purpose.
In another embodiment of the invention, the molding waste produced during operation of a typical green sand foundry is processed in essentially the same way as described above. However, in this instance the molding waste is first mechanically separated to produce a lighter and a heavier fraction. The lighter fraction contains most of the bentonite clay and organic components in the mold waste and therefore can be processed in the same way as described above, by itself or together with the bag house dust produced by the foundry, to recover its useful sand and bentonite clay values for making still additional green sand molds. The heavier fraction produced by mechanical separation is composed predominantly of sand. In accordance with still another feature of the invention, this reclaimed sand product can be made to exhibit a particle size and particle size distribution approximating that of prime sand by carrying out the mechanical separation process in an appropriate manner. Therefore, this heavier sand fraction, when appropriately made in accordance with the present invention, can replace at least some of the prime sand used in making new mold cores, thereby significantly reducing the foundry""s total demand for prime sand in its overall green sand molding process.